[SSSD] [PATCH] GPO: Fix crash with GPO and missing security descriptor

Lukas Slebodnik lslebodn at redhat.com
Wed Apr 29 07:38:31 UTC 2015


On (24/04/15 14:07), Jakub Hrozek wrote:
>On Fri, Apr 24, 2015 at 02:01:11PM +0200, Lukas Slebodnik wrote:
>> On (24/04/15 12:43), Jakub Hrozek wrote:
>> >On Thu, Apr 23, 2015 at 07:29:07AM -0400, Stephen Gallagher wrote:
>> >> On Thu, 2015-04-23 at 08:14 +0200, Lukas Slebodnik wrote:
>> >> > On (20/04/15 14:38), Stephen Gallagher wrote:
>> >> > > On Mon, 2015-04-20 at 08:53 +0200, Lukas Slebodnik wrote:
>> >> > > > ehlo,
>> >> > > > 
>> >> > > > attached patch fixes crash in
>> >> > > > https://fedorahosted.org/sssd/ticket/2629
>> >> > > > 
>> >> > > 
>> >> > > 
>> >> > > Nack. I'd rather we fixed the root of this problem. I did some 
>> >> > > digging
>> >> > > this afternoon and tracked the issue back to ad_gpo.c line 3499 (in
>> >> > > current master). If we get back a NULL result or num_results == 0,
>> >> > > then we just skip over this item in the list and start processing 
>> >> > > the
>> >> > > next one. Unfortunately, that leaves an item in the candidate_gpos
>> >> > > list that was never properly constructed.
>> >> > > 
>> >> > You are right.
>> >> > 
>> >> > We got a referral to GPO and therefore we do not find any attributes.
>> >> > 
>> >> > [sdap_sd_search_send] (0x0400): Searching entry [cn={2BA15B73-9524-
>> >> > 419F-B4B7-185E1F0D3DCF},cn=policies,cn=system,DC=example,DC=com] 
>> >> > using SD
>> >> > [sdap_print_server] (0x2000): Searching 10.1.1.14
>> >> > [sdap_get_generic_ext_step] (0x0400): calling ldap_search_ext with 
>> >> > [(objectclass=*)][cn={2BA15B73-9524-419F-B4B7-
>> >> > 185E1F0D3DCF},cn=policies,cn=system,DC=example,DC=com].
>> >> > [sdap_get_generic_ext_step] (0x1000): Requesting attrs: 
>> >> > [nTSecurityDescriptor]
>> >> > [sdap_get_generic_ext_step] (0x1000): Requesting attrs: [cn]
>> >> > [sdap_get_generic_ext_step] (0x1000): Requesting attrs: 
>> >> > [gPCFileSysPath]
>> >> > [sdap_get_generic_ext_step] (0x1000): Requesting attrs: 
>> >> > [gPCMachineExtensionNames]
>> >> > [sdap_get_generic_ext_step] (0x1000): Requesting attrs: 
>> >> > [gPCFunctionalityVersion]
>> >> > [sdap_get_generic_ext_step] (0x1000): Requesting attrs: [flags]
>> >> > [sdap_get_generic_ext_step] (0x2000): ldap_search_ext called, msgid =
>> >> > 14
>> >> > [sdap_process_result] (0x2000): Trace: sh[0x7f5d409a8c60], 
>> >> > connected[1], ops[0x7f5d409d2c10], ldap[0x7f5d409a9ed0]
>> >> > [sdap_process_result] (0x2000): Trace: ldap_result found nothing!
>> >> > [sdap_process_result] (0x2000): Trace: sh[0x7f5d409a8c60], 
>> >> > connected[1], ops[0x7f5d409d2c10], ldap[0x7f5d409a9ed0]
>> >> > [sdap_process_message] (0x4000): Message type: 
>> >> > [LDAP_RES_SEARCH_RESULT] 
>> >> > [sdap_get_generic_op_finished] (0x0400): Search result: 
>> >> > Referral(10), 0000202B: RefErr: DSID-0310063C, data 0, 1 access 
>> >> > points
>> >> >         ref 1: 'lzb.hq'
>> >> > 
>> >> > [sdap_get_generic_op_finished] (0x1000): Ref: 
>> >> > ldap://lzb.hq/cn=%7B2BA15B73-9524-419F-B4B7-
>> >> > 185E1F0D3DCF%7D,cn=policies,cn=system,DC=example,DC=com
>> >> > [ad_gpo_get_gpo_attrs_done] (0x0040): no attrs found for GPO; try 
>> >> > next GPO.
>> >> > 
>> >> > > Under what circumstances can the secinfo_dacl search return success
>> >> > > but with zero results? Is there a bug or a race here (such as the 
>> >> > > AD
>> >> > > server has updated the GPO since we got the list of candidate 
>> >> > > GPOs?).
>> >> > > How best to handle this?
>> >> > > 
>> >> > > With your patch here, it looks like we're assuming that it's okay 
>> >> > > to
>> >> > > just skip over this GPO. If that's the case, then what we really 
>> >> > > need
>> >> > > to be doing in ad_gpo_get_gpo_attrs_step() is to mark the gp_gpo as
>> >> > > being invalid and then after we've gone through them all, shrink 
>> >> > > the
>> >> > > array, removing all of the invalid entries. This will be more 
>> >> > > future-
>> >> > > proof, as it's not just the gpo_sd that is uninitialized here. 
>> >> > > Every
>> >> > > member of this gp_gpo is NULL except for the DN.
>> >> > > 
>> >> > > If it's *not* okay that we've gotten no results for this lookup 
>> >> > > (such
>> >> > > as in the race case; we don't want to be skipping over a GPO that
>> >> > > might properly be denying users), we may need to restart 
>> >> > > processing at
>> >> > > least a couple times to try to avoid the race and go offline if we
>> >> > > can't complete the processing (so we at least stick to our cached
>> >> > > rules).
>> >> > 
>> >> > I'm sorry I didn't noticed it in log files the first time.
>> >> > Now we know the root of problem.
>> >> > Which of your proposal do you prefer now?
>> >> > 
>> >> > LS
>> >> 
>> >> OK, I hadn't considered the referral scenario. (I wasn't actually 
>> >> aware that GPOs could *be* referred to elsewhere). Is this referral 
>> >> pointing to a different machine than the host is enrolled to? It's 
>> >> trying to reach a DN at lzb.hq that is identical to the one we just 
>> >> requested.
>> >> 
>> >> I'd really like to know how to reproduce the setup for this situation; 
>> >> is it inheriting a GPO from a parent domain somehow?
>> >> 
>> >> Neither of my above proposals are a proper solution for this case, 
>> >> unfortunately. The first one would make the crash go away, but if this 
>> >> GPO really exists and is restrictive, not processing it is *really 
>> >> bad*. Right now we're actually slightly lucky in that the crash has a 
>> >> net result of causing an access denial rather than erroneously 
>> >> resulting in a user login, but if we were to "solve" this problem the 
>> >> way Lukas's original patch intended (or my first alternative above), 
>> >> it would be introducing a security issue. So let's not do either of 
>> >> those.
>> >> 
>> >> The second option I suggested there is invalid as well, since the 
>> >> entry *exists*, it's just been converted to a referral. There are 
>> >> therefore only two safe options at this time:
>> >> 
>> >> 1) Quick-and-ugly: If we encounter a referral, skip straight to denial 
>> >> and loudly warn in the logs that this is a situation we are unable to 
>> >> handle yet. This will avoid the crash, but will leave the GPO behavior 
>> >> equivalently bad to the current state. Might be acceptable if this is 
>> >> only happening on a very small subset of systems. 
>> >
>> >I'm afraid we need to go this way now.
>> >
>> >> Users experiencing 
>> >> this might be able to set "ldap_referrals = True" on the affected 
>> >> systems to quietly solve the problem at the expense of all the other 
>> >> side-effects to that (particularly performance)
>> >
>> >I wouldn't even suggest enabling referrals. I've seen the LDAP provider
>> >misbehave completely with referrals enabled, openldap-libs is not really
>> >great at chasing them and LDAP provider would go offline etc..
>> >So enabling referrals doesn't just degrade performance, but also
>> >functionality..
>> >
>> >I also tested that using Global Catalog wouldn't help here, some
>> >attributes we require are not replicated to GC, such as gPCFileSysPath.
>> >
>> >Are you sure we're using the right connection to the right domain? We
>> >can select the right connection based on the DN that we know.
>> >I'm sorry, but I couldn't reproduce the bug myself.
>> >
>> Stephen wrote a reproducer into ticket
>> https://fedorahosted.org/sssd/ticket/2629
>
>I know, that's what I tried, but I couldn't get the setup to break. For
>some reason SSSD didn't examine the GPO from the parent domain if it was
>linked to a site the client was at..
>
I decided to break processing of GPOs when
attributes cannot be found.

I don't know why it was implemented in this way.
Is there a valid use case for skipping GPO with missing attributes?
Should I special case referral?

LS
-------------- next part --------------
>From 1a7a18764cff897047b93bf08a82d4dac68e913a Mon Sep 17 00:00:00 2001
From: Lukas Slebodnik <lslebodn at redhat.com>
Date: Wed, 29 Apr 2015 09:17:18 +0200
Subject: [PATCH] GPO: Do not ignore missing attrs for GPOs

We don't want to skip over a GPO that might properly be denying
users.

[sssd[be[a.foo.com]]] [sdap_sd_search_send] (0x0400):
    Searching entry [cn={2BA15B73-9524-419F-B4B7-185E1F0D3DCF},cn=policies,cn=system,DC=foo,DC=com] using SD
[sssd[be[a.foo.com]]] [sdap_get_generic_ext_step] (0x0400):
    calling ldap_search_ext with [(objectclass=*)][cn={2BA15B73-9524-419F-B4B7-185E1F0D3DCF},cn=policies,cn=system,DC=lzb,DC=hq].
[sssd[be[a.foo.com]]] [sdap_process_message] (0x4000):
    Message type: [LDAP_RES_SEARCH_RESULT]
[sssd[be[a.foo.com]]] [sdap_get_generic_op_finished] (0x0400):
    Search result: Referral(10), 0000202B: RefErr: DSID-0310063C, data 0, 1 access points
        ref 1: 'lzb.hq'
[sssd[be[a.foo.com]]] [sdap_get_generic_op_finished] (0x1000):
    Ref: ldap://foo.com/cn=%7B2BA15B73-9524-419F-B4B7-185E1F0D3DCF%7D,cn=policies,cn=system,DC=foo,DC=com
[sssd[be[a.foo.com]]] [ad_gpo_get_gpo_attrs_done] (0x0040):
    no attrs found for GPO; try next GPO.

Resolves:
https://fedorahosted.org/sssd/ticket/2629
---
 src/providers/ad/ad_gpo.c | 10 +++++++---
 1 file changed, 7 insertions(+), 3 deletions(-)

diff --git a/src/providers/ad/ad_gpo.c b/src/providers/ad/ad_gpo.c
index 23d67bc35ded5d93ee4cb63bf5ea6f5e938257cd..00f3caa0e81d35427885045417aafe8c0272caf1 100644
--- a/src/providers/ad/ad_gpo.c
+++ b/src/providers/ad/ad_gpo.c
@@ -3497,9 +3497,13 @@ ad_gpo_get_gpo_attrs_done(struct tevent_req *subreq)
     }
 
     if ((num_results < 1) || (results == NULL)) {
-        DEBUG(SSSDBG_OP_FAILURE, "no attrs found for GPO; try next GPO.\n");
-        state->gpo_index++;
-        ret = ad_gpo_get_gpo_attrs_step(req);
+        const char *gpo_dn = state->candidate_gpos[state->gpo_index]->gpo_dn;
+
+        DEBUG(SSSDBG_OP_FAILURE,
+              "No attrs found for GPO [%s].\n"
+              "It might be caused by referral which are not supported yet!\n",
+              gpo_dn);
+        ret = ERR_INTERNAL;
         goto done;
     }
     else if (num_results > 1) {
-- 
2.3.6



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